How can ES6 Classes be Used to Extend Functions and Access Instance Data?

Extending Function with ES6 Classes

In ES6, special objects can be extended, allowing inheritance from the Function object. While it's possible to call such objects as functions, implementing logic for this call can be challenging.

Passing Instance Data to Function Call

When calling a class as a function, this refers to the window object. To access instance data, two approaches are available:

1. Hardcoding: Force the super call to expect a code string containing the instance data.

class Smth extends Function {
  constructor(x) {
    super("return " + JSON.stringify(x) + ";");
  }
}

2. Using a Closure: Return a closure function that accesses instance variables.

class Smth extends Function {
  constructor(x) {
    function smth() { return x; };
    Object.setPrototypeOf(smth, Smth.prototype);
    return smth;
  }
}

Abstracting the Function Extension

A more generalized approach is to create an ExtensibleFunction class that handles the extension:

class ExtensibleFunction extends Function {
  constructor(f) {
    return Object.setPrototypeOf(f, new.target.prototype);
  }
}

This class can then be used to extend specific classes:

class Smth extends ExtensibleFunction {
  constructor(x) {
    super(() => { return x; }); // closure
  }
}

In summary, extending Function with ES6 classes allows for inheriting the function's behavior while customizing the call logic. Different approaches can be used to provide access to instance data when calling the extended function.

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